1. Field of the Invention
The present invention relates to the field of the technique concerning professional telecommunication systems, and more in particular, to a broad band digital radio receiver for multicarrier signals.
The use of the radiofrequency spectrum in telecommunication is governed by international standards assigning specific frequency bands to given services, either public or private. Inside these bands services are generally organized in order to exploit the band occupation at best, for instance, subdividing the same into a plurality of contiguous channels. A number of examples are available on this matter. A first example is represented by telephone radio links, where thousands of telephone channels are multiplexed among them, either in frequency or in time, in order to give a result contiguous within a microwave band. A second example is the Paneuropean telephone system, hereinafter referred to through the acronym GSM (Groupe Special Mobile), based on the time division use of even 174 carriers, 200 KHz spaced among them, modulated according to a GMSK scheme (Gaussian Minimum Shift Keying), and individually transmitted within a 35 MHz band, positioned around 900 MHz (EGSM). Reference to the GSM system is purposely made since, being the same as an essentially digital system, it results a preferred field of application according to the subject invention. The digital receiver definition means that it is designed to receive signals for which the parameter, or parameters, characterizing the modulated carriers, assume a discrete number of values; in the GSM, as in the most modern telecommunication systems, the carriers are modulated in an orthogonal way, starting from a modulating signal consisting of bursts of information or synchronization bits.
A problem arising in the modern transceivers is in fact that of the conversion of the reception analog signal into a digital format, from which the original burst has to be obtained through appropriate processing with the DSP techniques (Digital Signal Processing). The classical implementation scheme of radio receivers operating in the field of the present invention foresees at least an intermediate frequency conversion stage, followed by a demodulator and an analog-to-digital converter (A/D) of the demodulated signal. The reasons inducing to the intermediate frequency conversion of the signal received are multiple, among which the main one is undoubtedly that of an improved and more easy selectivity of the receiver. Of course, the conversion to filter the signal falling in a so-called xe2x80x9cimagexe2x80x9d band at radio frequency is a speculation compared to one versus the frequency foI of the local oscillator governing the intermediate frequency converter. Such a filtering is generally very complex, due to the close distance usually present among adjacent radio channels. A second problem arising, is the conversion speed of the A/D converter, since it depends on the bandwidth of the signal to be processed. The above mentioned speed corresponds to the sampling frequency fs of a sampler of the analog signal preceding the A/D converter. The frequency fs must be equal to at least the double of the maximum frequency included in the BW band of the signal to be converted, as defined by the Nyquist proposition, which represents a non negligible burden in the case of broad band signals, just like multicarrier ones.
2. Background Art
In order to double the band to be processed by the A/D converter, a functional diagram is shown in FIG. 1 of a multicarrier receiver, simplified for description sake, to the case of only two carriers representing two adjacent channels in a comprising BW band. The receiver of FIG. 1 is enabled to halve the sampling frequency fs and can be obtained through the sole application of the conventional knowledge of the skilled in the art.
Referring first to FIG. 1, a radiofrequency stage including a low-noise amplifier RFAMP for a RF input signal consisting of two carriers having fc1 and fc2 frequency, respectively, is orthogonally modulated by the information conveyed by the relevant channels CH1 and CH2 associated to the same. The signal coming out from RFAMP is equally shared over two branches leading to the input of two relevant band pass filters PBAND1 and PBAND2 having width BW/2, sharing the whole RF band. The signals coming out from the filters reach two first inputs of relevant mixers MIX1 and MIX2, the second inputs of which are reached by two sinusoidal signals of local oscillator, respectively, having foI1=fc1xe2x88x92BW/4 and foI2=fc2xe2x88x92vBW/4 frequencies. Thanks to the particular values of foI1 and foI2, the two channels CH1 and CH2 are included in the 0 to BW/2 band. The signals are filtered by two low-pass filters, not shown in the figure, eliminating the 2fol1 and 2fol2 components reaching two A/D blocks operating at fs=BW frequency. The digital signals coming out from the A/D blocks reach two DDC blocks representing some numeric demodulators in quadrature. For the detail of these blocks, reference will be made to the description of the following figures. The in phase component I1 and the in quadrature component Q1 of the demodulated signal concerning channel CH1 are present at the two outputs of block DDC1 likewise the in phase component I2 and the in quadrature component Q2 of the demodulated signal concerning channel CH2 are present at the two outputs of block DDC2. The above-mentioned components are sent to a detector block, not shown in the figure, giving back the starting information. The diagram of FIG. 1 can be extended to a receiver for more than two channels, simply adding as much DDC blocks as are the new channels.
As it can be noticed from the previous description, in the receiver of FIG. 1 the A/D converters operate at halved speed compared to those used in the receivers mentioned above. However, this advantage versus the background art is soon made vain by the cost of the two high selectivity, radiofrequency filters PBAND1 and PBAND2 and by the need to equip two local oscillators.
Object of the present invention is to overcome the drawbacks of the background art and of the receiver of FIG. 1, and to indicate a method for the implementation of a broad band radio receiver for multicarrier signals with orthogonal modulation.
The above object is solved by the present invention which is addressed to a method for the implementation of a broad band receiver for a signal (z1(t)) consisting of a plurality of equispaced carriers, orthogonally modulated by information conveyed by relevant channels in order to carry out a radiofrequency multicarrier signal, the method comprising in sequence the following steps:
(a) direct demodulation (DEMI/Q) of the radiofrequency filtered signal (z1xe2x80x2(t)) by multiplication of the signal for two local phase quadrature carriers (cosxcfx890t, xe2x88x92sinxcfx890t), whose frequency corresponds to a central value of the radiofrequency multicarrier signal spectrum, obtaining a demodulated signal having a relevant spectrum in the lower half of the base band (BW) where pairs of channels (CH4, CH5; . . . ; CH1, CH8) in symmetric positions at the two sides of the local phase quadrature carriers are superimposed;
(b) broad band filtering in base band in phase component (z2xe2x80x2(t)) and in quadrature component (z3xe2x80x2(t)) that correspond to the demodulated signal for the suppression of additional components outside an interest band;
(c) sampling of the components broad band filtered in base band (z2xe2x80x2(t)), making use of a sampling frequency equal to the bandwidth (BW) of the multicarrier signal, and subsequent analog-to-digital conversion (A/D) of the sampled components, obtaining first digital in phase and quadrature components;
(d) digital demodulation (DEM4, DEM5; . . . ; DEM1, DEM8) of the first digital components by multiplication for pairs of relevant phase quadrature sinusoidal digital signals having a frequency equal to the value of a center band of pairs of channels (CH4, CH5; . . . ; CH1, CH8) previuosly superimposed in step (a), obtaining in coincidence with the pairs of phase quadrature sinusoidal digital signals due to multiplication, quartets of numeric values (z4xe2x80x2(t)4/5, z5xe2x80x2(t)4/5, z6xe2x80x2(t)4/5, z7xe2x80x2(t)4/5; . . . ; z4xe2x80x2(T)1/8, z5xe2x80x2(t)1/8, z6xe2x80x2(t)1/8, z7xe2x80x2(t)1/8) that can be analytically expressed through linear systems, each of four equations in four unknown values corresponding to amplitudes of second digital in phase (I4, I5; . . . ; I1, I8) and in quadrature (Q4, Q5; . . . ; Q1, Q8) components belonging to the quartets of numeric values;
(e) solution of the linear systems (RSOM) obtaining the above-mentioned components (I4, Q4; I5, Q5; . . . ; I1, Q1; I8 Q8), with each single channel of the pairs of channels, thus eliminating a phase and amplitude dyssymmetry in base band due to the superimposition of channels in step (a).
An additional object of the invention is a radio receiver implemented according to the above mentioned process. The receiver employs A/D converters operating at halved speed, like what previously described referring to the background art mentioned first. However, compared to the receiver of FIG. 1 it employs only one radiofrequency local oscillator and one sole radiofrequency band pass filter having not too high selectivity, since the problem to filter the image band does not exist, compared to the two high selectivity filters and the two local oscillators required to implement the above mentioned solution of the known type. The economy is lightly penalized by the addition of N/2 numeric networks for the reconstruction of the in phase and in quadrature components of the single demodulated channels of each pair of channels that resulted superimposed in base band. As it can be noticed, in the receiver according to present invention, the most expensive analog portion is reduced to the minimum necessary extent, in favor of the digital portion, simpler, reliable and less expensive.
The receiver object of the present invention is able to correctly operate and to show the advantages listed above, provided that the electrical behavior of the mixers inserted in the two branches in quadrature of the analog demodulator result perfectly symmetric. In the contrary instance, undesired components would be originated in the demodulated signal, which would prevent the network that eliminates the equivocation in base band to obtain the original values of the in phase and in quadrature components of the demodulated carriers. Incidentally, the receiver includes second demodulators with numeric mixers not introducing any unbalancing. The mentioned inconvenience can be overcome selecting for the analog demodulator a couple of mixers, simultaneously obtained in a same manufacturing procedure, with high coupling degree of the physical parameters, assured by the manufacturer.
Would this not be enough, it is however possible to modify the receiver according to a modified process particularly useful in case of multicarrier signals for channels subject to a wide level dynamic, such as for instance the signals used in the mobile communication systems. The variant differs from the main process due to the fact that the broad band filtering of the demodulated signal is a band pass filtering suppressing from the spectrum of the signal in base band the components in a condition of null frequency and that introduces the following additional steps completely independent from the main sequence:
measurement of the amplitude and phase dissymmetry degree on the two branches of an analog demodulator performing the direct demodulation from radiofrequency in base band, obtaining correction factors;
introduction of the correction factors in the linear systems of equations, obtaining still linear modified systems having the same number of equations;
solution of the modified systems, obtaining for each single channel of the pairs the above-mentioned components in phase and in quadrature, without unbalancing otherwise due to the dissymmetries on the two branches of the analog demodulator, further comprising the steps of:
measuring the amplitude and phase dissymmetry degree on two branches of an analog demodulator (DEMI/Q performing said direct demodulation from radiofrequency to base band;
introducing four correction factors (a, b, c, d), which depend on the preceding measurement, in said linear systems of equations, obtaining modified systems, still linear, having the same number of equations;
solving said modified systems (RSCOR), in corresponding quartets of numeric values (z4xe2x80x2(t)4/5, z5xe2x80x2(t)4/5, z6xe2x80x2(t)4/5, z7xe2x80x2(t)4/5; . . . ; z4xe2x80x2(t)1/8, z5xe2x80x2(t)1/8, z6xe2x80x2(t)1/8, z7xe2x80x2(t)1/8) modified unknown amplitudes of the second digital in phase and in quadrature components, without unbalancing distortion, otherwise due to said dissymmetries on the two branches of said analog demodulator.